1. Field of the Invention
The invention relates to vehicle transmissions, i.e. the series of members transmitting the rotational movement of the engine to the wheels of the vehicle, which members comprise a gear-box, a rear axle and a double cardan shaft for transferring the torque from the gear-box to the rear axle while allowing relative transverse movements thereof.
It relates more particularly, among these transmissions, to those equipped with an electric eddy current retarder exerting a braking torque thereon at desired times, said retarder comprising on the one hand an annular inductor stator cantilevered on the casing of the gearbox of the vehicle by means of an appropriate frame and on the other hand a rotor comprising two induced ferromagnetic material discs which surround the stator and are both supported by a part itself cantilevered on a stub shaft inside the casing, each disc being fixed for this purpose, by means of a crown of arms forming ventilation fins, to a ring which is in its turn fixed to said part.
2. Description of the Prior Art
In known embodiments of the transmissions of the kind in question, the part on which the two rings are mounted is generally a special insert plate which is fixed both to a "gear-box flange" fixed on a stub shaft inside the casing of the gearbox and, on the axial side opposite said stub shaft, to an end flange of the contiguous cardan joint.
It should in fact be noted that it is not possible in practice to fit the rotor crowns directly on the gearbox flange, for at least the crown disposed on the gear-box side would then have to be mounted before the flange is positioned on the gear-box.
Now, such positioning is provided by forced axial fitting together of complementary splines formed respectively on the stub shaft inside the gear-box and on a cylindrical sleeve extending the flange such force fitting only being able to be carried out with the flange not loaded.
The special insert plate must have a certain thickness since it must be able to transmit the whole of the drive torque of the vehicle from the cardan to the gear-box flange.
Thus, it has the double drawback of relatively high weight and axial dimension.
The last drawback may be serious in the case of very short transmissions, i.e. in which a limited space is available between the output of the gear-box and the input of the rear axle.
This is in particular the case for certain modern buses or coaches with rear engine, in which the overall length of the engine/gear-box assembly and that of the rear axle, particularly of hypoid type, increase without the respective positions of these members being modified.
In such a case, it may be vital to be able reduce, even by a few centimeters only, the extension of the transmission due to the fitting of an electric retarder of the above kind thereon, the possibility of adopting such a retarder being sometimes directly related to such a reduction.
To reduce the extension mentioned, I have proposed in U.S. patent application Ser. No. 07/337/485 filed June 18, 1990 to enlarge the gearbox flange transversely and to fix the rotor rings of the retarder on the periphery of this enlarged flange via a tubular insert extending axially from this periphery on the side opposite the gearbox.
The end flange of the cardan joint concerned is then fixed directly to the gearbox flange, inside said tubular insert.
Such a construction is interesting, in particular because the insert, only transmitting retarding torques and not drive torques may be formed by a very light part, comprising essentially a tube section reinforced with internal radial ribs.
However, this approach assumes that the opening freed in the centre of the retarder has a large diameter, this opening having to be sufficient to receive concentrically the tubular insert and, thereinside, the flange of the cardan joint.